1. Field of the Invention
The present invention relates in general to heat pipes and, more particularly, to a method of sealing a heat pipe.
2. Description of Related Art
With the ever-increasing density and high power of electronic components, e.g., memories and logic arrays in high speed computers, the problem of heat generation by electronic components in close proximity to one another on electronic circuit cards has become of increasing concern to industry. In response, over the past several years heat pipes have been developed to cool electronic circuit cards. Typically, a number of heat pipes are formed and placed into a metal substrate which is bonded to a circuit card. In its conventional form, a heat pipe is a closed tube or chamber of various shapes whose inner surfaces are lined with a porous capillary wick. The wick is saturated with a working fluid. The heat pipe has an evaporator section where the heat pipe absorbs heat, and also has a condensor section where heat is released to a heat sink in contact with that section of the pipe. In operation, heat absorbed by the evaporator section causes liquid to evaporate from the wick. The resultant vapor is transferred within the tube to the condensor section of the heat pipe where it condenses releasing the heat of vaporization to a heat sink. The capilliary action of the wick pumps the condensed liquid back to the evaporator section for reevaporation. The process will continue as long as working fluid is contained within the heat pipe.
However, too often the liquid in the heat pipe chamber is lost due to a break in the heat pipe seal. The ability to reliably and effectively seal heat pipes has been sought by the industry for many years, because if the fluid within the heat pipe is lost due to a leak in the heat pipe the equipment cooled by the heat pipe could be subject to great heat damage. Several means of sealing heat pipes have evolved over the last couple of years.
In one conventional arrangement, for example, a heat pipe includes a hollow tube with end caps inserted into each end of the tube. One end cap has a hole therethrough with a copper pinchoff tube brazed to the hole. The heat pipe is purged and filled with the proper working fluid using the copper tube. To seal the heat pipe the copper tube is pinched shut using a roller pinch off tool. See, for example, Dunn & Reay, Heat Pipes 154 (3rd Ed. 1982). However, the rollers of the pinch off tool get close to the braze and may crack the braze during pinch off. Additionally, after being sealed the fragile copper tube protrudes outwardly a short distance from the end cap, and therefore is very susceptible to breakage and consequently loss of fluid. In order to adequately protect this protruding copper tube, a cover must be placed over the end cap and copper tube. The end cap cover and copper tube disadvantageously consumes a large portion of the condenser section at the end of the heat pipe. Both reliability and efficiency of the heat pipe fabricated by this technique are limited.
In an attempt to improve upon this design, the copper tube has been attached directly to the side of the heat pipe tube instead of to the end cap. A copper tube is welded to a hole within the side of the heat pipe tube, and the heat pipe tube chamber is purged and filled with working fluid using this copper tube. After filling the heat pipe with fluid the copper tube is pinched shut to seal the tube. As with the above described process, the braze can be cracked during pinch off. Furthermore, this sealing technique is disadvantageous in that a portion of the copper tube extends outwardly from the side of the heat pipe. In this arrangement the fragile copper tube has no cover and is very susceptible to breakage. Additionally, the placement of the copper pinchoff tube on the side of the heat pipe tube hampers expulsion of noncondensable gases during purging. Furthermore, because the copper tube protrudes outwardly from the side of the heat pipe, heat pipes formed by this technique cannot be placed adjacent to each other.
Consequently, there is a need in the industry for a means of sealing a heat pipe which is economically accomplished and provides a strong and reliable seal.